Drug delivery to the brain

The blood-brain barrier (BBB) strictly controls the transport of substances into the brain thereby maintaining brain homeostasis and protecting the brain from harmful compounds [92, 93]. This, however, also results in limited access of most drugs thereby preventing efficient treatment of many brain diseases such as brain cancer, Alzheimer’s disease and others [92, 93]. Understanding the structure and function of the BBB and transport mechanisms across the BBB will be useful when designing new nanoparticle-based drug delivery systems and delivery strategies.

1.3.1 The blood-brain barrier

The BBB roughly consists of endothelial cells, pericytes, astrocyte end feet, immune cells and the basement membrane, and is graphically illustrated in Figure 6 [92, 94]. Brain capillaries are aligned with endothelial cells which are strongly bound together by tight junctions sealing the paracellular cleft and restricting paracellular transport [92, 94]. Tight junctions consist of transmembrane proteins such as claudins, occludins, junction adhesion molecules (JAM), and are connected to the cytoskeleton through intracellular adaptor proteins such as zonula occludens (ZO) and others [92, 94]. Besides the brain specific tight junctions, brain capillary endothelial cells (BCECs) are also tightly bound together by adherens junctions, an ubiquitous junctional complex in vasculature [95]. BCECs are morphologically and functionally different from other endothelial cells. They express higher levels of ABC transporters, contain higher number of mitochondria and show lower levels of leukocyte adhesion molecules [96]. The endothelial cell layer is supported by pericytes which are located at the abluminal side and are embedded in the basement membrane. Pericytes are capable of regulating blood flow by modulating capillary diameter and are important for maintaining BBB integrity [97]. Besides other functions, pericytes also aid angiogenesis and have been observed to have phagocytic capabilities [98, 99]. The basement membrane in which the pericytes are embedded provides mechanical stability to the vessel and a scaffold for cellular components of the BBB thereby also regulating intercellular communication by acting as a physical barrier [94, 96]. Astrocytes can be found in the brain parenchyma and are in direct contact with the basement membrane by their endfeet. They provide a link between the vasculature and neuronal circuit. Additionally, astrocytes are mainly known for their role in maintaining the BBB microenvironment by monitoring electrochemical activity, innate immune regulation and control levels of parenchymal water and metabolites [92, 94, 96].

Figure 6. Graphical illustration of the blood-brain barrier (BBB). The BBB roughly consists of brain capillary endothelial cells, pericytes, astrocyte end feet, immune cells and the basement membrane. The brain capillary endothelial cells are strongly bound together by tight junctions thereby sealing the paracellular cleft and restricting paracellular transport.

Perivascular macrophages and microglia are the two primary types of immune cells present in the BBB. Figure created with BioRender.

Perivascular macrophages and microglia are the two primary types of immune cells present in the BBB. Both have the ability to phagocytose cellular debris, pathogens and waste products thereby mediating immune response and maintaining the BBB integrity [96, 100]. Both immune cells are active players in several brain disorders and will affect accumulation and distribution of drug and drug carriers in brain tissue [96].

1.3.2 Transport pathways across the blood-brain barrier

Compounds can enter the brain parenchyma by para- or transcellular transport.

Since paracellular transport is highly limited due to tight junctions sealing the inter-endothelial cleft, transport across the BBB is mainly restricted to transcellular pathways. Several transcellular pathways for drug delivery purposes can be exploited at the BBB and these are illustrated in Figure 7.

Gases (e.g. oxygen, carbon dioxide) and small lipophilic molecules (< 400 Da) can cross the BBB by transcellular diffusion [6, 93]. Transport proteins facilitate the transport across the BBB of larger molecules such as glucose (carrier-mediated transcytosis). A variety of macromolecules such as transferrin and Figure 7. Graphical illustration of the different transport pathways across the blood-brain barrier. Figure is inspired by [6] and created with BioRender.

insulin binds to receptors on the BBB, following receptor-mediated endocytosis and release of the compound at the abluminal side [93, 101-104]. Cationic molecules (e.g. polymers, albumin) may interact with the negatively charged cell membrane and endocytosed through adsorptive endocytosis [102, 105].

Whereas the aforementioned transport mechanisms can be utilized for drug delivery across the BBB, drugs face the risk of getting expelled back into the lumen by drug efflux pumps which are overly expressed on the plasma membrane of BCECs opposed to regular endothelial cells [90, 106, 107].

1.3.3 Approaches to overcome the blood-brain barrier

Both non-invasive and invasive approaches to circumvent the BBB have been investigated for the last decades. Different designs of drugs, drug carriers and nanoparticles able to exploit the discussed transport mechanisms, have been developed. Unfortunately, due to the restrictive nature of the BBB, low delivery efficiencies are often obtained such that high intravenous doses are required to achieve relevant therapeutic concentrations at the target site often resulting in adverse systemic effects [93, 108]. The permeability of the BBB can temporarily be increased by using disruptive agents such as hyperosmotic solutions, vasodilators or chemical agents [109, 110]. However, these agents offer poor spatial control over the site of increased permeability. Additionally, many of the chemical agents used are toxic and may cause neuronal damage [109, 111].

Delivery of drugs through the nasal epithelium has been explored as well but showed low delivery efficiency [112-114]. Intracerebral or intraventricular injections were used for direct delivery of therapeutic agents to the target region, but these are highly invasive and thus unfavourable [115]. A more promising and non-invasive approach is the use of focused ultrasound in combination with intravascular microbubbles [116]. With this approach, reversible, temporal and local disruption of the BBB can be achieved which has demonstrated to facilitate delivery of chemotherapeutics, antibodies, nanocarriers and stem cells across the BBB [32, 111, 117-124].